Japanese Laid-Open Patent Publication No. 5-42063 proposes a device which clings to a surface and can be moved along the surface including: four sets of vertical expansion and contraction means; one set of width direction expansion and contraction means disposed for each one of the four sets of vertical expansion and contraction means; clinging units of which is disposed for each one of the width direction expansion and contraction means; and first putting forward/backward means interposed between the clinging unit and the width direction expansion and contraction means, the device being characterized in that each of the clinging units is selectively set at least in the following three states: (1) a clinging and movable state in which the device can be moved along the surface and in close contact with the surface; (2) a clinging and locked state in which the device clings to the surface in a locked manner; and (3) a non-clinging and movable state in which the device is separated from the surface and can be moved along the surface.
Japanese Laid-Open Patent Publication No. 5-42063 mentioned above proposes a device which clings to a surface and can be moved along the surface in which, in the case where a protruding bar such as a window glass frame exists on the surface, and when the protruding bar extends in the horizontal direction and the vertical direction and has a number of crossing portions, the device can be moved by stepping over the protruding bar in either of the horizontal direction or the vertical direction. In the above-described device, the cleaning direction is supposed to be only the horizontal direction. The cleaning in the vertical direction is not supposed.
In the above-described device, when the device is moved by stepping over the window glass frame extending in the horizontal direction, two of the four clinging units are separated from the window glass, i.e., only the half of the clinging units included in the device is held by vacuum on the window glass, so that the device has insufficient stability in vacuum holding. Thus, for example, on the surface of a window glass which forms an angle of 90 degrees or less with the ground, i.e., on the surface of a window glass in a so-called over-hanging condition, the device involves a problem that it cannot be moved by stepping over the window glass frame extending in the horizontal direction.